Fixture for use in a coating operation

ABSTRACT

This invention relates to a fixture for use in a physical vapor deposition coating operation which comprises a support structure  14  comprising a circular base member  10 , a circular top member  11  opposite the circular base member  10 , and a plurality of structural members  12  joining said top member  11  to said base member  10 ; a plurality of panel members  13  aligned in a vertical direction around the outer periphery of said support structure  14  forming a cylinder-like structure; said panel members  13  including a plurality of apertures for holding workpieces  19  and  35  to which a coating is to be applied; and said apertures positioned on said panel members  13  so that said workpieces  19  and  35  are aligned in a staggered vertical direction. This invention also relates to a method for simultaneously coating a plurality of workpieces  19  and  35 , such as gas turbine compressor blades and vanes, with erosion resistant coatings using the fixture of this invention.

FIELD OF THE INVENTION

This invention relates to a fixture for use in a physical vapordeposition coating operation and a method for simultaneously coating aplurality of workpieces, such as gas turbine compressor rotor blades andstator vanes, with erosion resistant coatings.

BACKGROUND OF THE INVENTION

Frequently, aircraft, tank and helicopter gas turbine engines areoperated in a desert environment where the gas turbine compressor rotorblades and stator vanes are exposed to erosive media such as sand anddust. These erosion effects lead to increased fuel consumption,horsepower loss, higher overall operating temperatures, and can causedamage to compressor and turbine hardware. Erosion resistant coatingssuch as TiN, TiCN, TiZrN, TiZrCN, TiAlN and TiAlCN, applied by cathodicarc physical vapor deposition, can be used to prolong the life ofcompressor airfoils in a sand erosion environment. See, for example U.S.Pat. No. 5,071,693.

Physical vapor deposition is a line of sight coating process. Substratesto be coated need to be manipulated in the vapor to achieve uniformcoverage. Sections of the substrate that do not require a coating needto be masked adequately.

Turbine compressor airfoils have an airfoil section which extendsoutward to pump the air into the combustion chamber. The portion of theturbine airfoil opposite the airfoil portion is used to attach theairfoil to the disk or rotor part of the engine, which is not in theflow of the air, and therefore is not in need of protection from erosiveeffects. This portion of the turbine airfoil often has the shape of adovetail, which is assembled into dovetail slots on the disk or rotorportion of the engine. Hence, it is frequently referred to as thedovetail.

The walls of the dovetail portion of the turbine airfoil contact thewalls of the dovetail slots of the disk or rotor. After a long period oftime or rotating at high speeds, the dovetail walls exhibit afatigue-related phenomenon referred to as fretting. Fretting has beenfound to be exacerbated by coatings applied to the airfoil portion ofthe turbine airfoil. Thus, in order to achieve the desired properties inthe various portions of the turbine airfoil to maximize the life of theturbine airfoil, it has been necessary to devise methods to properlycoat the airfoil portion of the turbine airfoil without affecting thedovetail portion of the turbine airfoil.

Physical vapor deposition is extensively used to apply ceramic thermalbarrier coatings on turbine airfoils by electron beam evaporation.Extensive work has been done to design processes and fixturing to applya uniform coating on a variety of airfoil sizes and shapes.

U.S. Pat. No. 5,997,947 discloses a modular, rotisserie type coatingfixture for use in electronic beam physical vapor deposition (EBPVD)coating processes. The fixture includes a support structure and meansattached to the support structure for allowing it to be rotated about afirst axis. The fixture further includes cassette means within thesupport structure for holding one or more workpieces to be coated. Thecassette means are joined to the support structure by spindles whichallow the cassette means to rotate about a second axis substantiallyparallel to the first axis and thereby allow each workpiece being coatedto rotate about its longitudinal axis. The cassette means support eachworkpiece so that surfaces of the airfoil to be coated are maintainedsubstantially parallel to the coating source.

As indicated above, cathodic arc physical vapor deposition can be usedto apply erosion resistant coatings. Cathodic arc physical vapordeposition is, in the first order, a line of sight deposition process.Vapor created by the cathodic arc discharge on the cathode surfacepropagates basically in a straight line from the cathode surface towardsthe workpieces. Second order effects in vapor propagation includescattering of the ionized vapor in the chamber gas atmosphere, whichcreates some throwing power around corners and into cavities and moresignificantly, electrostatic attraction of the highly ionized vapor, andthus enhanced coating build-up on edges and sharp contours of theworkpieces such as airfoil tips and leading and trailing edges of theairfoils. The predominant deposition feature however, when coating thesurface of flat workpieces such as the platform and airfoil ofcompressor blades and vanes, is the line of sight propagation of thevapor. To provide the maximum coating thickness on the workpieces suchas the lower part of the airfoil and on the platform, the view betweenthe cathode surface and the workpiece surfaces should not be obstructed.

There continues to be a need in the art for coating fixtures for use ina physical vapor deposition coating operation which allow forsimultaneous coating of a plurality of workpieces and which promote theproduction of high quality coatings. The coating fixtures should permitcoating application to only those portions of the workpieces requiringprotective coatings while protecting those portions not requiring aprotective coating. Further, the coating fixtures should be reusable andrelatively inexpensive to fabricate.

SUMMARY OF THE INVENTION

This invention relates to a fixture for use in a coating operation whichcomprises:

a support structure comprising a circular base member, a circular topmember opposite the circular base member, and a plurality of structuralmembers joining said top member to said base member;

a plurality of panel members aligned in a vertical direction around theouter periphery of said support structure forming a cylinder-likestructure, said panel members secured on said circular top member andsaid circular base member;

said panel members including a plurality of apertures for holdingworkpieces to which a coating is to be applied, said apertures facingoutwardly away from said support structure, and said aperturesconfigured for receiving a portion of said workpieces that does notrequire coating;

said panel members comprising a plurality of plate members, at leastsome of said plate members providing a barrier for segregating theinternal volume of said apertures from the volume external to saidapertures and for securing said workpieces in said apertures; and

said apertures positioned on said panel members so that said workpiecesare aligned in a vertical direction and a portion of the workpieces thatrequires coating extends into the volume external to said apertures anda portion of the workpieces that does not require coating is maintainedwithin the internal volume of said apertures, so that there isessentially no direct communication between the internal volume of theapertures and the external volume of the apertures across said barrier.

This invention also relates to a method for simultaneously coating aplurality of workpieces comprising:

(i) providing a fixture comprising:

a support structure comprising a circular base member, a circular topmember opposite the circular base member, and a plurality of structuralmembers joining said top member to said base member;

a plurality of panel members aligned in a vertical direction around theouter periphery of said support structure forming a cylinder-likestructure, said panel members secured on said circular top member andsaid circular base member;

said panel members including a plurality of apertures for holdingworkpieces to which a coating is to be applied, said apertures facingoutwardly away from said support structure, and said aperturesconfigured for receiving a portion of said workpieces that does notrequire coating;

said panel members comprising a plurality of plate members, at leastsome of said plate members providing a barrier for segregating theinternal volume of said apertures from the volume external to saidapertures and for supporting and securing said workpieces in saidapertures; and

said apertures positioned on said panel members so that said workpiecesare aligned in a vertical direction and a portion of the workpieces thatrequires coating extends into the volume external to said apertures anda portion of the workpieces that does not require coating is maintainedwithin the internal volume of said apertures so that there isessentially no direct communication between the internal volume of theapertures and the external volume of the apertures across said barrier;

(ii) loading and securing workpieces into said apertures of said panelmembers;

(iii) positioning said fixture onto a rotatable drive system in acoating chamber having a source of coating material; and

(iv) operating the drive system to cause the fixture to rotate andsimultaneously coating by physical vapor deposition that portion of saidworkpieces that extends into the volume external of said apertures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a portion of a fixture showing acylindrical support structure and three panel members secured thereto.

FIG. 2 is an exploded view of a portion of a panel member used forcoating rotor blades.

FIG. 3 is an exploded view of a portion of a panel member used forcoating stator vanes.

FIG. 4 is a side view of a portion of a fixture positioned on a rotarytable in a coating chamber.

FIG. 5 is a photograph of a coating fixture showing a cylindricalsupport structure and panel members secured thereto.

FIG. 6 is a photograph of a coating fixture showing panel memberssecured thereto.

FIG. 7 is a photograph of a portion of a panel member showing astaggered airfoil arrangement.

DETAILED DESCRIPTION OF THE INVENTION

As indicated above, the fixture of this invention comprises:

a support structure 14 comprising a circular base member 10, a circulartop member 11 opposite the circular base member 10, and a plurality ofstructural members 12 joining said top member 11 to said base member 10;

a plurality of panel members 13 aligned in a vertical direction aroundthe outer periphery of said support structure 14 forming a cylinder-likestructure, said panel members 13 secured on said circular top member 11and said circular base member 10;

said panel members 13 including a plurality of apertures for holdingworkpieces to which a coating is to be applied, said apertures facingoutwardly away from said support structure, and said aperturesconfigured for receiving a portion of said workpieces that does notrequire coating;

said panel members 13 comprising a plurality of plate members, at leastsome of said plate members providing a barrier for segregating theinternal volume of said apertures from the volume external to saidapertures and for securing said workpieces in said apertures; and

said apertures positioned on said panel members 13 so that saidworkpieces are aligned in a vertical direction and a portion of theworkpieces that requires coating extends into the volume external tosaid apertures and a portion of the workpieces that does not requirecoating is maintained within the internal volume of said apertures, sothat there is essentially no direct communication between the internalvolume of the apertures and the external volume of the apertures acrosssaid barrier. In a preferred embodiment, the fixture of this inventionis specifically designed for use in physical vapor deposition coating ofcompressor airfoils with TiN or similar erosion resistant coatings.

Key features of the fixture of this invention include among others: avertically arranged cylindrical fixture that holds from about 12 or lessto about 36 or more, preferably about 18 to about 30, more preferablyabout 22 to about 26, elongated flat panel fixtures; panel members aredesigned to securely hold from 1 to about 4 or more rows, preferably 2rows, of from about 5 or less to about 25 or greater, preferably about10 to about 20, more preferably about 10 to about 15 airfoils and tomask the dovetail; panel members allow easy mix and match of similarsized but different part types in one coating run; vertical arrangementof airfoils minimizes the exposure to dust before and during coating;staggered airfoil arrangement minimizes the coating loss by selfshadowing; precision masking of the dovetail is accomplished byprecision machined masks and additional shields; locking of parts with acircumferential dovetail is accomplished by a moveable clamp plate;locking of parts with a ledge type dovetail is accomplished by a drop-insandwich type concept; and panels are designed such as to enable aqueouscleaning or vapor degreasing after assembly thereby eliminating the needfor human contact with the parts after cleaning.

The coating fixture depicted in FIG. 1 shows a cylindrical supportstructure 14 with panel members 13 aligned in a vertical directionaround the outer periphery of the cylindrical support structure 14. FIG.1 shows a circular base member 10, circular top member 11 and structuralmembers 12, i.e., rod-like members, joining the circular base member 10to the circular top member 11. The circular top member 11 in FIG. 1includes a means for receiving an end of said panel members 13 and thebase member 10 also includes a means for receiving an end of said panelmembers 13, thereby securing the panel members 13 to the supportstructure 14. In an embodiment, the top member 11 can include a raisedlip or ledge for receiving the panel member 13 that can include a hookedend permitting it to fit onto the raised lip or ledge. The base member10 can include a groove for receiving an end of said panel members 13.Other suitable means for affixing panel members 13 to the cylindricalsupport structure 14 may include hooks, bolts, nuts, screws, wire,clips, pins and the like. Conventional securing means may be employedwith the panel members 13 and cylindrical support structure 14. Thecoating fixture can contain from about 12 or less to about 36 or greaterpanel members 13 depending on the size of the coating fixture and thesize of the panel members 13.

The size of the fixture is not narrowly critical and can vary over awide range. However, as depicted in FIG. 4, the fixture should be ofappropriate size so as to permit its positioning onto a rotatable drivesystem 40 in a coating chamber 41 and to permit the drive system 40 tobe operated, thereby allowing the fixture to rotate. The circumferenceof the fixture can typically range from about 75 centimeters or less toabout 250 centimeters or greater, preferably from about 90 to about 200centimeters, and more preferably from about 90 to about 125 centimeters.The diameter of the fixture can typically range from about 20centimeters or less to about 100 centimeters or greater, preferably fromabout 25 to about 75 centimeters, and more preferably from about 30 toabout 50 centimeters. The height of the fixture can typically range fromabout 20 centimeters or less to about 100 centimeters or greater,preferably from about 25 to about 75 centimeters, and more preferablyfrom about 30 to about 50 centimeters.

As shown in FIG. 2, the panel members useful for coating rotor blades 19comprise (i) a base mask plate member 20 having no apertures for holdingsaid workpieces 19, said base mask plate member 20 providing a basebarrier for segregating the internal volume of said apertures from thevolume external to said apertures, (ii) a base plate member 21 having aplurality of apertures for holding said workpieces 19, said base platemember 21 positioned above said base mask plate member 20, (iii) a clampplate member 22 having a plurality of apertures for holding saidworkpieces 19, said apertures being aligned with the apertures of saidbase plate member 21, said clamp plate member 22 positioned above saidbase plate member 21, and said clamp plate member 22 providing a topbarrier for segregating the internal volume of said apertures from thevolume external to said apertures and securing said workpieces 19 insaid apertures, (iv) a rotor blade support plate member 23 having aplurality of apertures for holding said workpieces 19, said aperturesbeing aligned with the apertures of said base plate member 21 and clampplate member 22, said rotor blade support plate member 23 positionedabove said clamp plate member 22, (v) a rotor blade mask plate member 24having a plurality of apertures for holding said workpieces 19, saidapertures being aligned with the apertures of said base plate member 21,clamp plate member 22 and rotor blade support plate member 23, saidrotor blade mask plate member 24 positioned above said rotor bladesupport plate member 23, and (vi) fastening means 25, 27, 28 and 29 forsecuring together said base mask plate member 20, base plate member 21,clamp plate member 22, rotor blade support plate member 23 and rotorblade mask plate member 24.

Illustrative fastening means for securing together said base mask platemember 20, base plate member 21, clamp plate member 22, rotor bladesupport plate member 23 and rotor blade mask plate member 24 include,for example, screws, spacers, nuts, and the like as shown in FIG. 2.Other suitable fastening means may include bolts, pins, wire, clips andthe like. Conventional fastening means may be employed with the panelmembers.

The clamp plate member 22 is important for securing the rotor bladeworkpieces 19 in the panel members. As shown in FIG. 2, the apertures ofsaid clamp plate member 22 are removably interlockable with theapertures of said base plate member 21 and said rotor blade supportplate member 23. The clamp plate member 22 includes a fastening meansaperture that is larger than the fastening means aperture of the baseplate member 21 and rotor blade support plate member 23, therebyallowing the clamp plate member 22 to be adjusted to secure the rotorblade workpieces 19 in the panel members.

The size of the panel members useful for coating rotor blades is notnarrowly critical and can vary over a wide range. However, the panelmembers should be of appropriate size so as to permit their positioningand securing to the support structure. The panel members can typicallyrange in length from about 20 centimeters or less to about 100centimeters or greater, preferably from about 25 to about 75centimeters, and more preferably from about 30 to about 50 centimeters.The panel members can typically range in width from about 2 centimetersor less to about 10 centimeters or greater, preferably from about 2 toabout 6 centimeters, and more preferably from about 2 to about 5centimeters. The panel members can typically range in thickness fromabout 0.1 centimeter or less to about 1 centimeter or greater,preferably from about 0.2 to about 0.6 centimeters, and more preferablyfrom about 0.2 to about 0.5 centimeters.

As shown in FIG. 3, the panel members useful for coating stator vanes 35comprise (i) a base plate member 30 having no apertures for holding saidworkpieces 35, said base plate member 30 providing a base barrier forsegregating the internal volume of said apertures from the volumeexternal to said apertures, (ii) a root mask plate member 31 having aplurality of apertures for holding said workpieces 35, said root maskplate member 31 positioned above said base plate member 30, (iii) amasking plate member 32 having a plurality of apertures for holding saidworkpieces 35, said apertures being aligned with the apertures of saidroot mask plate member 31, said masking plate member 32 positioned abovesaid root mask plate member 31, and said root mask plate member 31 andsaid masking plate member 32 providing a top barrier for segregating theinternal volume of said apertures from the volume external to saidapertures and securing said workpieces 35 in said apertures, and (iv)fastening means 33 and 34 for securing together said base plate member30, root mask plate member 31 and masking plate member 32.

Illustrative fastening means for securing together said base platemember 30, root mask member 31 and masking plate member 32 include, forexample, screws, spacers, nuts, and the like as shown in FIG. 3. Othersuitable fastening means may include bolts, pins, wire, clips and thelike. Conventional fastening means may be employed with the panelmembers.

The size of the panel members useful for coating vanes is not narrowlycritical and can vary over a wide range. However, the panel membersshould be of appropriate size so as to permit their positioning andsecuring to the support structure. The panel members can typically rangein length from about 20 centimeters or less to about 100 centimeters orgreater, preferably from about 25 to about 75 centimeters, and morepreferably from about 30 to about 50 centimeters. The panel members cantypically range in width from about 2 centimeters or less to about 10centimeters or greater, preferably from about 2 to about 6 centimeters,and more preferably from about 2 to about 5 centimeters. The panelmembers can typically range in thickness from about 0.1 centimeters orless to about 1 centimeter or greater, preferably from about 0.2 toabout 0.6 centimeters, and more preferably from about 0.2 to about 0.5centimeters.

The panel members allow easy mix and match of similar sized butdifferent workpiece types in one coating run. For example, the fixturemay include some panel members with gas turbine compressor rotor bladesand other panel members with gas turbine compressor stator vanes.Preferably, each panel member contains all the same workpieces. Thedesign of the coating fixtures of this invention enables efficientcoating of a large number of airfoils with high productivity and yield.First time coating yield is very high because of minimized coatingdefects caused by particulate contamination. This high yield isaccomplished through a vertical optimized airfoil orientation asdescribed below.

The coating fixture orients the airfoils with the tip facing outward ona cylindrical surface as shown in FIGS. 1, 5 and 6. To provide theflexibility to mix and match airfoils in one coating run, airfoils ofthe same part type are grouped on part specific panel members which areaffixed to a cylindrical support structure. A panel member can holdtypically from 1 to about 4 or more rows, each row containing from about5 or less to about 25 or more apertures for holding workpieces. Theairfoils are oriented vertically so as to minimize the collection ofdust on the surface and thereby prevent coating defects. The airfoilsare preferably spaced and arranged so as to maximize the coating buildup and to minimize self shadowing. This is achieved by airfoil rowspacing and a staggered airfoil arrangement as described herein and asshown in FIG. 7.

In the coating fixtures of this invention, the dovetail is maskedagainst coating build-up which, if not masked, would otherwise lead towear issues and to undesired dimensional issues in the rotor/statorassembly process. All masking plates are precision machined to provideprecision masking of the dovetail with minimum overspray.

Additionally, as described below, all fixtures are designed so as toallow aqueous cleaning or vapor degreasing after assembly by arrangingappropriated draining holes for the cleaning liquid. This conceptensures no contact with the surface to be coated after final cleaning.

The number of apertures for holding workpieces on a panel member is notnarrowly critical and can vary over a wide range depending on the typeand size of the cathodic arc coating equipment used, as well as the sizeof the airfoils. A panel member typically has from 1 to about 4 rows ormore, preferably 2 rows, of apertures for holding workpieces, each rowhaving from about 5 or less to about 25 or more apertures depending onthe size of the workpieces. Preferably, a panel member has 2 rows offrom about 5 to about 20 apertures for holding workpieces.

The fixture of this invention permits the coating of workpieces, such asgas turbine compressor rotor blades and stator vanes, which can havevarious dovetail designs or configurations, such as circumferential,ledge, axial and others. With respect to the dovetail design orconfiguration of airfoils, the dovetail needs to be masked againstcoating build-up so as to prevent wear issues and undesired dimensionalissues in the rotor/stator assembly process. In accordance with thisinvention, the panel members for airfoils with a circumferential oraxial dovetail, typically rotor blades, are designed as a multilayersheet metal construction consisting of a masking plate 24, rotor bladesupport plate 23, clamp plate 22, base plate 21 and base masking plate20 as shown in FIG. 2. The panel members for airfoils with an axialdovetail may be designed to include a mounting block which also acts asa dovetail mask. The panel members for airfoils with a dovetail with aledge, typically stator vanes, are designed as a multilayer sheet metalconstruction consisting of a masking plate 32, root mask plate 31 andbase plate 30 as shown in FIG. 3. All masking plates are precisionmachined to provide precision masking of the dovetail with minimumoverspray.

As indicated above, in a preferred embodiment of this invention, thecoating fixture comprises a cylindrical drum composed of verticallyarranged panel members. Another variation of this invention is tocompose a cylinder by vertically stacked circular or ring-shaped panelmembers. This variation would arrange the airfoils in the same way as inthe panel member embodiment and would also allow also mixing andmatching of workpieces.

In accordance with this embodiment, this invention relates to a fixturefor use in a coating operation which comprises:

a support structure comprising a circular base member, a circular topmember opposite the circular base member, and a plurality of structuralmembers joining said top member to said base member;

a plurality of circular panel members aligned in a horizontal directionaround the outer periphery of said support structure forming acylinder-like structure;

said panel members including a plurality of apertures for holdingworkpieces to which a coating is to be applied, said apertures facingoutwardly away from said support structure, and said aperturesconfigured for receiving a portion of said workpieces that does notrequire coating;

said panel members comprising a plurality of plate members, at leastsome of said plate members providing a barrier for segregating theinternal volume of said apertures from the volume external to saidapertures and for securing said workpieces in said apertures; and

said apertures positioned on said panel members so that said workpiecesare aligned in a horizontal direction and a portion of the workpiecesthat requires coating extends into the volume external to said aperturesand a portion of the workpieces that does not require coating ismaintained within the internal volume of said apertures, so that thereis essentially no direct communication between the internal volume ofthe apertures and the external volume of the apertures across saidbarrier.

The size of the circular panel members useful for coating blades is notnarrowly critical and can vary over a wide range. However, the circularpanel members should be of appropriate size so as to permit theirpositioning and securing to the support structure. The circular panelmembers can typically range in circumference from about 75 centimetersor less to about 250 centimeters or greater, preferably from about 80 toabout 200 centimeters, and more preferably from about 90 to about 150centimeters. The circular panel members can typically range in widthfrom about 2 centimeters or less to about 10 centimeters or greater,preferably from about 2 to about 6 centimeters, and more preferably fromabout 2 to about 5 centimeters.

The number of apertures for holding workpieces on a circular panelmember is not narrowly critical and can vary over a wide range. Acircular panel member typically has from 1 to about 4 rows or more,preferably 2 rows, of apertures for holding workpieces, each row havingfrom about 12 or less to about 200 or more apertures depending on thesize of the workpieces. Preferably, a circular panel member has 2 rowsof from about 12 to about 100 apertures for holding workpieces.

In the embodiment involving circular panel members, the circular topmember of the support structure may include a means for receiving a sideof said circular panel members and the base member may also include ameans for receiving a side of said circular panel members, therebysecuring the circular panel members to the support structure. In anembodiment, the top member and base member can include a groove forreceiving a side of said circular panel members. Other suitable meansfor affixing circular panel members to the cylindrical support structuremay include bolts, nuts, screws, pins, wire, clips and the like.Conventional securing means may be employed with the circular panelmembers and support structure.

As with the vertically arranged panel members, the airfoils on circularpanel members are oriented vertically so as to minimize the collectionof dust on the surface and thereby prevent coating defects. The airfoilsare preferably spaced and arranged so as to maximize the coating buildup and to minimize self shadowing. This is achieved by airfoil rowspacing and a staggered airfoil arrangement as described herein and asshown in FIG. 7.

The components of the coating fixture of this invention may be formedfrom any suitable material known in the art. However, the fixturematerial should be electrically conductive so as to provide current flowto the workpieces to be coated in the ion etching and plating processsteps of the coating operation. For example, all the components of thecylindrical support structure as well as all the components of the panelmembers, may be formed from a metallic material such as stainless steel,e.g., SST 304 and Inconel.

All fixtures are designed so as to enable aqueous cleaning or vapordegreasing after assembly by arranging appropriated draining holes forthe cleaning liquid. This concept ensures no contact with surface to becoated after final cleaning. Particularly, the fixture is designed suchthat it allows easy draining of apertures of an aqueous cleaningsolution. This enables cleaning of the fixture assembly after fixturingthe workpieces to remove any contamination on the surface of theworkpieces. Surface contamination of the workpieces can be introducedduring fixturing by human contact and the like. Preferably, the baseplate member used in panel members for coating vanes as depicted in FIG.3 has a plurality of draining holes for removal of the aqueous cleaningsolution.

As also indicated above, this invention relates to a method forsimultaneously coating a plurality of workpieces comprising:

(i) providing a fixture comprising:

a support structure 14 comprising a circular base member 10, a circulartop member 11 opposite the circular base member 10, and a plurality ofstructural members 12 joining said top member 11 to said base member 10;

a plurality of panel members 13 aligned in a vertical direction aroundthe outer periphery of said support structure 14 forming a cylinder-likestructure, said panel members 13 secured on said circular top member 11and said circular base member 10;

said panel members 13 including a plurality of apertures for holdingworkpieces to which a coating is to be applied, said apertures facingoutwardly away from said support structure, and said aperturesconfigured for receiving a portion of said workpieces that does notrequire coating;

said panel members 13 comprising a plurality of plate members, at leastsome of said plate members providing a barrier for segregating theinternal volume of said apertures from the volume external to saidapertures and for securing said workpieces in said apertures; and

said apertures positioned on said panel members 13 so that saidworkpieces are aligned in a vertical direction and a portion of theworkpieces that requires coating extends into the volume external tosaid apertures and a portion of the workpieces that does not requirecoating is maintained within the internal volume of said apertures sothat there is essentially no direct communication between the internalvolume of the apertures and the external volume of the apertures acrosssaid barrier;

(ii) loading and securing workpieces into said apertures of said panelmembers 13;

(iii) positioning said fixture onto a rotatable drive system 40 in acoating chamber 41 having a source of coating material; and

(iv) operating the drive system 40 to cause the fixture to rotate andsimultaneously coating by physical vapor deposition that portion of saidworkpieces that extends into the volume external of said apertures.

As depicted in FIG. 4, the fixture is positioned in a physical vapordeposition coating chamber 41 upon a rotatable drive system 40, e.g.,rotary table. Arc cathodes 42 are positioned on the side walls of thecoating chamber 41. The rotatable drive system 40 is operated causingthe fixture to rotate. The workpieces are simultaneously coated byphysical vapor deposition. FIG. 6 shows a fixture filled with panelmembers with each panel member filled with workpieces.

This invention also relates to a method for simultaneously coating aplurality of workpieces comprising:

(i) providing a fixture comprising:

a support structure comprising a circular base member, a circular topmember opposite the circular base member, and a plurality of structuralmembers joining said top member to said base member;

a plurality of circular panel members aligned in a horizontal directionaround the outer periphery of said support structure forming acylinder-like structure;

said panel members including a plurality of apertures for holdingworkpieces to which a coating is to be applied, said apertures facingoutwardly away from said support structure, and said aperturesconfigured for receiving a portion of said workpieces that does notrequire coating;

said panel members comprising a plurality of plate members, at leastsome of said plate members providing a barrier for segregating theinternal volume of said apertures from the volume external to saidapertures and for securing said workpieces in said apertures; and

said apertures positioned on said panel members so that said workpiecesare aligned in a horizontal direction and a portion of the workpiecesthat requires coating extends into the volume external to said aperturesand a portion of the workpieces that does not require coating ismaintained within the internal volume of said apertures, so that thereis essentially no direct communication between the internal volume ofthe apertures and the external volume of the apertures across saidbarrier;

(ii) loading and securing workpieces into said apertures of said panelmembers;

(iii) positioning said fixture onto a rotatable drive system in acoating chamber having a source of coating material; and

(iv) operating the drive system to cause the fixture to rotate andsimultaneously coating by physical vapor deposition that portion of saidworkpieces that extends into the volume external of said apertures.

Physical vapor deposition in the coating chamber can be conducted byconventional methods known in the art. See, for example, U.S. Pat. No.5,071,693, the disclosure of which is incorporated herein by reference.Illustrative erosion resistant coating system that can be used with thecoating fixtures of this invention include TiN, TiCN, TiZrN, TiZrCN,TiAlN, TiAlCN, and the like. Physical vapor deposition can be used toapply the erosion resistant coatings by reactive evaporation orsputtering, for example, by cathodic arc evaporation, hollow cathodereactive electron beam evaporation, reactive sputtering, and the like.

To provide the maximum coating thickness on the workpieces such as thelower part of the airfoil and on the platform, the view between thecathode surface and the workpiece surfaces should not be obstructed. Inthe preferred arrangement of vertically aligning the workpieces in rowson panels and on a rotating fixture, the workpieces can shadowthemselves if the rows are spaced too closely. This self shadowing canbe minimized, when the rows are spaced sufficiently wide to allow vaporreach the workpieces in a row without obstruction of the vapor flux bythe workpieces in a neighboring row.

To further enhance the vapor flux to the platforms and lower parts ofthe airfoil of the workpieces, it is advantageous to space theworkpieces in vertical direction and to stagger the workpieces locationfrom row to row such that the vapor can pass through the workpieces ofone row and can reach the platform and lower parts of the airfoils ofthe staggered workpieces in the next row.

In a preferred embodiment, row spacing of one to two times of theairfoil length (platform to tip), a workpiece spacing in verticaldirection of one to two times of the chord width of the airfoil, and analternating staggered arrangement of the workpieces in the rows canprovide basically the same deposition rate at the platform and lowerparts of the airfoil of the workpiece as a free standing workpiecewithout any self shadowing.

In an embodiment of this invention, a multilayer nitride-containingcoating is produced on the workpieces by a method comprising:

(a) placing a fixture containing the workpieces to be coated on arotatable drive system 40 in a chamber 41, said fixture comprising:

a support structure 14 comprising a circular base member 10, a circulartop member 11 opposite the circular base member 10, and a plurality ofstructural members 12 joining said top member 11 to said base member 10;

a plurality of panel members 13 aligned in a vertical direction aroundthe outer periphery of said support structure 14 forming a cylinder-likestructure, said panel members 13 secured on said circular top member 11and said circular base member 12;

said panel members 13 including a plurality of apertures for holdingworkpieces to which a coating is to be applied, said apertures facingoutwardly away from said support structure, and said aperturesconfigured for receiving a portion of said workpieces that does notrequire coating;

said panel members 13 comprising a plurality of plate members, at leastsome of said plate members providing a barrier for segregating theinternal volume of said apertures from the volume external to saidapertures and for securing said workpieces in said apertures; and

said apertures positioned on said panel members 13 so that saidworkpieces are aligned in a vertical direction and a portion of theworkpieces that requires coating extends into the volume external tosaid apertures and a portion of the workpieces that does not requirecoating is maintained within the internal volume of said apertures, sothat there is essentially no direct communication between the internalvolume of the apertures and the external volume of the apertures acrosssaid barrier;

(b) adding to the chamber 41 a titanium based target and anitrogen-containing gas mixture;

(c) operating said drive system 40 to cause the fixture to rotate andsimultaneously evaporating the titanium from the titanium-based targetto produce a titanium vapor to react with the nitrogen in thenitrogen-containing gas mixture to form a titanium nitride-containinglayer of a desired nitrogen content on that portion of the workpiecesthat extends into the volume external of said apertures;

(d) changing the ratio of nitrogen to titanium in step (c) to formanother titanium nitride-containing layer on the coated workpieces thathas a nitrogen content of at least 2 atomic percent of nitrogendifferent than the nitrogen contained in the previously depositedcoating; and

(e) repeating step (d) at least once to form a multilayer coating of atleast two layers in which at least one layer contains at least 2 atomicpercent of nitrogen different than the nitrogen contained in an adjacentlayer. The ratio of nitrogen to titanium can be changed by altering thecurrent, changing the flow of nitrogen or a combination of both.

In another embodiment of this invention, a multilayer nitride-containingcoating is produced on the workpieces by a method comprising:

(a) placing a fixture containing the workpieces to be coated on arotatable drive system 40 in a vapor deposition chamber 41, said fixturecomprising:

a support structure 14 comprising a circular base member 10, a circulartop member 11 opposite the circular base member 10, and a plurality ofstructural members 12 joining said top member 11 to said base member 10;

a plurality of panel members 13 aligned in a vertical direction aroundthe outer periphery of said support structure 14 forming a cylinder-likestructure, said panel members 13 secured on said circular top member 11and said circular base member 10;

said panel members 13 including a plurality of apertures for holdingworkpieces to which a coating is to be applied, said apertures facingoutwardly away from said support structure, and said aperturesconfigured for receiving a portion of said workpieces that does notrequire coating;

said panel members 13 comprising a plurality of plate members, at leastsome of said plate members providing a barrier for segregating theinternal volume of said apertures from the volume external to saidapertures and for securing said workpieces in said apertures; and

said apertures positioned on said panel members 13 so that saidworkpieces are aligned in a vertical direction and a portion of theworkpieces that requires coating extends into the volume external tosaid apertures and a portion of the workpieces that does not requirecoating is maintained within the internal volume of said apertures, sothat there is essentially no direct communication between the internalvolume of the apertures and the external volume of the apertures acrosssaid barrier;

(b) adding to the vapor deposition chamber 41 an anode and atitanium-based cathode along with a nitrogen-containing gas mixture;

(c) operating said drive system 40 to cause the fixture to rotate andsimultaneously applying a voltage across the cathode and anode toestablish a current to effect evaporation of the titanium from thetitanium-based cathode to produce a titanium vapor to react with thenitrogen in the nitrogen-containing gas mixture to form a titaniumnitride-containing layer of a desired nitrogen content on that portionof the workpieces that extends into the volume external of saidapertures;

(d) changing the ratio of nitrogen to titanium in step (c) to formanother titanium nitride-containing layer on the coated workpieces thathas a nitrogen content of at least 2 atomic percent of nitrogendifferent than the nitrogen contained in the previously depositedcoating; and

(e) repeating step (d) at least once to form a multilayer coating of atleast two layers in which at least one layer contains at least 2 atomicpercent of nitrogen different than the nitrogen contained in an adjacentlayer. The ratio of nitrogen to titanium can be changed by altering thecurrent, changing the flow of nitrogen or a combination of both.

Preferably, the nitrogen-containing gas mixture can be argon-nitrogen,krypton-nitrogen, helium-nitrogen, xenon-nitrogen, neon-nitrogen, andthe like.

The nitride-containing compound of each layer may have an atomic percentof nitrogen from 33% to 55%, preferably from 40% to 50%, and morepreferably from 42% to 50%. The nitride-containing compound of anadjacent layer may have an atomic percent of nitrogen from 33% to 45%,and preferably from 39% to 42%. The nitrogen content in one layer shouldbe at least 2 atomic percent of nitrogen different than the nitrogencontent in an adjacent layer. The multilayer coating can be deposited byusing conventional process techniques such as physical vapor deposition.The changing of the ratio of nitrogen to titanium for the alternatelamellar layers will interrupt the grain growth process of the coatingcompound so that the grain size of the compound is no larger than thethickness of the individual layers.

The preferred coating can comprise a layer of titanium nitride having anatomic percent of nitrogen from 40% to 55% alternating with a layer oftitanium nitride having an atomic percent of nitrogen from 33% to 45%and wherein at least one layer has at least 2 atomic percent of nitrogendifferent than the nitrogen content in each adjacent layer on oppositesides of such layer. Titanium nitride with the above ranges of nitrogencan have the same orientation and crystallographic structure with asmall difference in lattice spacings so that coherent interfaces betweenthe layers can be expected to produce a high toughness characteristic.

The coating fixture of this invention facilitates the deposition of highquality and evenly distributed coatings. Hardness and toughness of amultilayer coating are closely related to compositions and spacings oflayers. The individual layer thickness and overall thickness of themultilayer coating depends on specific applications. For systemsapplications requiring high toughness, the layer with the smallercontent of nitrogen should be from 1 to 20 times thicker than the layercontaining the higher content of nitrogen. An overall coating thicknessof from 5 microns up to 30 microns thick is generally adequate for mosterosion applications.

The thickness of the individual layers can vary greatly, for example,between 0.1 and 5 microns thick, preferably about 1 micron thick. Thenumber of layers should be at least two so that at least one layer willhave an atomic percent of nitrogen of 2% more or 2% less than thenitrogen content in the adjacent layer. The number of layers of anitride-containing compound forming the coating can vary from two up toany number desired for a particular application. Generally, 5 to 50layers, preferably 15 to 40 layers, of the coating would be suitable formost applications.

In some applications, it may be advisable to have a relatively thickfirst layer of the nitride-containing compound to support subsequentmultilayers of the coating and/or a thick top layer to provide a hardertop surface. The multilayer coating is ideally suited for coatingworkpieces made of materials such as titanium, steel, aluminum, nickel,cobalt, alloys thereof, and the like.

In another embodiment, this invention relates to multilayer coatedworkpieces, e.g., rotor blades and stator vanes, that are coated with atleast two layers of a nitride-containing compound with each layercontaining at least one additive from the group comprising titanium,zirconium, titanium alloys and zirconium alloys and wherein at least onelayer contains at least 2 atomic percent of nitrogen different than thenitrogen contained in the adjacent layer. The layer can also contain atleast one element from the group comprising aluminum, vanadium,molybdenum, niobium, iron, chromium and manganese. Preferably, themultilayer coated workpiece will comprise three or more layers in whichat least one layer will contain at least 2 atomic percent of nitrogendifferent than the nitrogen contained in each of the adjacent layers.

Various modifications and variations of this invention will be obviousto a worker skilled in the art and it is to be understood that suchmodifications and variations are to be included within the purview ofthis application and the spirit and scope of the claims.

EXAMPLES

Multilayer titanium nitride coatings were deposited on rotor blade andstator vane workpieces using the coating fixture shown in FIG. 6 and aphysical vapor deposition arc evaporation process. Panel members wereloaded with rotor blades or stator vanes and then secured on thecylindrical support structure of the coating fixture. The coatingfixture was then placed in a vapor deposition chamber. Beforedeposition, the vapor deposition chamber was evacuated to a pressurebelow 7×10⁻⁴ Pa and then backfilled with argon to 0.7 Pa. The rotorblade and stator vane workpieces to be coated were sputtered to removesurface contaminants. The coating fixture was rotated at about 1-10rotations per minute during the coating process. Subsequently, a directcurrent arc was activated across a titanium cathode and a chamber whichacts as an anode to evaporate Ti from the Ti cathode in an argon-N₂ gasmixture at an overall pressure between 1.3 to 4.8 Pa. The ionized Tivapor reacted with N₂ ions and then formed titanium nitride coatings onthe workpieces. The layered structure of the coating was formed byaltering the N₂ gas flow rates during the deposition. Typically, thecoating consisted of a lamellar structure of A and B nitride layers inwhich the thickness of the B layer was greater than that of the A layer.The nitrogen concentration in the A layer was generally greater than thenitrogen content in the B layer. The deposition produced a multilayercoating of 15 alternating A and B layers. The resulting thickness of thecoating at the airfoil close to the tip was about 17-20 microns, at theairfoil close to the platform was about 7-9 microns, and on the platformwas about 9-10 microns.

1. A fixture for use in a coating operation which comprises: a supportstructure comprising a circular base member, a circular top memberopposite the circular base member, and a plurality of structural membersjoining said top member to said base member; a plurality of panelmembers aligned in a vertical direction around the outer periphery ofsaid support structure forming a cylinder structure, said panel memberssecured on said circular top member and said circular base member; saidpanel members including a plurality of apertures for holding workpiecesto which a coating is to be applied, said apertures facing outwardlyaway from said support structure, and said apertures configured forreceiving a portion of said workpieces that does not require coating;said panel members comprising a plurality of plate members, at leastsome of said plate members providing a barrier for segregating theinternal volume of said apertures from the volume external to saidapertures and for securing said workpieces in said apertures; and saidapertures positioned on said panel members so that said workpieces arealigned in a vertical direction and a portion of the workpieces thatrequires coating extends into the volume external to said apertures anda portion of the workpieces that does not require coating is maintainedwithin the internal volume of said apertures, so that there isessentially no direct communication between the internal volume of theapertures and the external volume of the apertures across said barrier.2. The fixture of claim 1 wherein said circular base member comprises acircular band member, said circular top member comprises a circular bandmember, and said structural members comprise rod members.
 3. The fixtureof claim 1 wherein said top member includes a means for receiving an endof said panel members and said base member includes a means forreceiving an end of said panel members, thereby securing said panelmembers to said support structure.
 4. The fixture of claim 1 comprisingat least one or more panel members.
 5. The fixture of claim 1 comprisingpanel members having at least 1 or more rows in which each row comprisesat least one or more apertures for holding workpieces.
 6. The fixture ofclaim 1 wherein said apertures are positioned on said panel members sothat said workpieces are aligned in a staggered vertical direction. 7.The fixture of claim 1 wherein said panel members include the sameworkpieces.
 8. The fixture of claim 1 wherein one or more panel membersinclude workpieces different from other panel members.
 9. The fixture ofclaim 1 wherein said panel member comprises (i) a base mask plate memberhaving no apertures for holding said workpieces, said base mask platemember providing a base barrier for segregating the internal volume ofsaid apertures from the volume external to said apertures, (ii) a baseplate member having a plurality of apertures for holding saidworkpieces, said base plate member positioned above said base mask platemember, (iii) a clamp plate member having a plurality of apertures forholding said workpieces, said apertures being aligned with the aperturesof said base plate member, said clamp plate member positioned above saidbase plate member, and said clamp plate member providing a top barrierfor segregating the internal volume of said apertures from the volumeexternal to said apertures and securing said workpieces in saidapertures, (iv) a blade support plate member having a plurality ofapertures for holding said workpieces, said apertures being aligned withthe apertures of said base plate member and clamp plate member, saidblade support plate member positioned above said clamp plate member, (v)a blade mask plate member having a plurality of apertures for holdingsaid workpieces, said apertures being aligned with the apertures of saidbase plate member, clamp plate member and blade support plate member,said blade mask plate member positioned above said blade support platemember, and (vi) fastening means for securing together said base maskplate member, base plate member, clamp plate member, blade support platemember and blade mask plate member.
 10. The fixture of claim 9 in whichthe aperture of said clamp plate member is removably interlockable withthe apertures of said base plate member and said blade support platemember.
 11. The fixture of claim 9 wherein the workpieces are airfoilshaving a circumferential dovetail.
 12. The fixture of claim 9 whereinthe workpieces are airfoils having an axial dovetail.
 13. The fixture ofclaim 1 wherein said panel member comprises (i) a base plate memberhaving no apertures for holding said workpieces, said base plate memberproviding a base barrier for segregating the internal volume of saidapertures from the volume external to said apertures, (ii) a root maskplate member having a plurality of apertures for holding saidworkpieces, said root mask plate member positioned above said base platemember, (iii) a masking plate member having a plurality of apertures forholding said workpieces, said apertures being aligned with the aperturesof said root mask plate member, said masking plate member positionedabove said root mask plate member, and said root mask plate member andsaid masking plate member providing a top barrier for segregating theinternal volume of said apertures from the volume external to saidapertures and securing said workpieces in said apertures, and (iv)fastening means for securing together said base plate member, root maskplate member and masking plate member.
 14. The fixture of claim 13wherein the workpieces are vanes having a dovetail with a ledge.
 15. Thefixture of claim 1 wherein said workpieces are coated by physical vapordeposition.
 16. The fixture of claim 15 wherein said workpieces arecoated by physical vapor deposition of an erosion resistant coatingsystem.
 17. The fixture of claim 16 wherein said erosion resistantcoating system is selected from TiN, TiCN, TiZrN, TiZrCN, TiAlN andTiAlCN.
 18. The fixture of claim 17 wherein said erosion resistantcoating system comprises a multilayer erosion resistant coating system.19. The fixture of claim 16 wherein said erosion resistant coatingsystem comprises a multilayer erosion resistant coating system.